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"A picture is worth more than a thousand words". True, but a large picture will make your webpage slower, which will affect your SEO in a negative way. And eat away at your servers space, megabyte after megabyte.

There are several ways to remedy such a behaviour, but one way is to use image compression services to save space. With online services or programs on your computer you can remove unnecessary information and compress images with sometimes up to 80% gain.